The present invention relates to a plate heat exchanger.
The plate heat exchanger according to the invention is in the first place intended for evaporation of a product, such as for instance caustic soda, sugar etc. through condensation of a heating medium which can be formed by the first medium. The product is comprised by or consist of the second medium which is transported through the second plate interspaces. Due to the evaporation process, vacuum, or a very low pressure, will prevail in the plate interspaces. This low pressure requires a strengthening sheet, or a so called vacuum sheet, which is provided between the second end plate, the so called pressure plate, and the proximate outermost heat exchanger plate. The strengthening sheet has to be thick, for instance more than 5 mm, in order to resist the low pressure without being deformed. The thickness is determined by, among other things, the size of the portholes. The larger the portholes the thicker strengthening sheet is needed. In the cases that the media, which are transported through the plate heat exchanger, comprise corroding or degrading substances, such as caustic soda, the strengthening sheet in addition has to be manufactured in a resistant material, advantageously the same material in which the heat exchanger plates are manufactured, for instance a material consisting of at least one of Ti, Ni and alloys thereof, or more noble stainless steel alloys. This means that the strengthening sheet becomes very expensive.